Why is the red diode in my RGB led burning out?

Question

I have been working on a simple circuit that uses a 10mm RGB led. I have had the same problem again with the red diode dying out after maybe a day. This time I thought I fixed it, but I guess I'm wrong. I have a 330 ohm resistor connected to it, which based on ohms law and the data sheet should be enough, but it just seems to stop working when I turned it on. What might be the problem of this.

Information on the circuit: It's basically a simple thermometer that gives you a idea what the tempature is based off of the color of the RGBled. It uses a attiny85 to control everything and a LM 35 tempature sensor 9 volts is the input to a LM7805 regulator which then feeds to the chip. Then from the outputs of the chip a 330 ohm resistors is connected from the chip to all the anodes.

The data sheet of the LED: http://cdn.sparkfun.com/datasheets/Components/General/YSL-R1047CR4G3BW-F8.pdf

Answer 1

The current through the red LED will be higher than the others due to it's lower voltage. However given a 5V supply it should still only be about 10ma which is well within the ratings given on the LED datasheet.

So either you got a crap batch of LEDs or you have something else wrong in your circuit. Some possibilities I would consider.

1. Your voltage regulator is not actually operating correctly due to missing ground connection, missing capacitors or some other issue.

2. Your resistors are not the values you think they are. In my experiance resistor color codes are all too easy to misread and I would strongly suggest testing them with a multimeter.

I would also suggest you post a full schematic of your circuit so we can see if there are any design errors.

Answer 2

The RED LED needs to be compensated for its lower operating voltage, relative to GRN and BLU.

If you use a 3V supply voltage for the LEDs, kicked down from your original 9V, and a 75 ohm in-line, current limiter, all LEDS will operate as triggered.

Since the RED LED is almost half the voltage of GRN (or BLU), a circuit designed specifically to GRN and BLU and operating at their maximum will Open the RED (or cause it to Flicker incessantly, or turn ORG...voltage/limiter dependant). If designed toward the GRN minimum, the RED will likely not even "fire".

For an LED circuit such as yours, I recommend you design to the lowest common denominator and chemical phenominae, such as the RED LED: 3V, 75♤ will work and maintain luminance and life for all three RGB.

Answer 3

If you check the "Forward Voltage" section on the LED's datasheet, you'll see that Red only holds 1.8V, while Green holds 3.0V and blue, 2.9V.

So, while using the same 330ohm value resistor for each of the Green & Blue leads, in order to keep within the same current parameters, you'll need to up your series resistor for the Red to somewhere around 500ohms (assuming 5V Vout from control chip).

Right now, your G & B series resistors are holding a voltage drop of ~Vout-3V across them, while the R series resistor has a drop of Vout-1.9, with corresponding higher current throughput if you use a same-value resistor.